Carbon nanotubes (CNTs) are cylidrical tubes of carbon molecules formed entirely of Sp bonds that are similar to the carbon bonds in graphite. The electrical and mechanical properties of CNTs render these structures potentially useful for a number of applications in fields such as nanotechnology, electronics and structural materials.
A number of techniques have been developed to produce CNTs, with an increasing focus and effort in maximizing production of high quality CNTs having sufficiently low levels of impurities. Examples of known CNT production techniques include arc discharge, laser ablation, high pressure carbon monoxide (HiPco), and chemical vapor deposition (CVD).
In certain methods, CNTs are formed on a supporting transition metal surface upon diffusion of catalytically formed carbon atoms and subsequent nucleation and growth on the surface. For example, in a CVD process, a carbon-containing gas (e.g., acetylene, ethylene or ethanol) is reacted with metal catalyst particles or a metal catalyst substrate (typically iron, nickel or cobalt) at sufficiently high temperatures (e.g., at about 600° C. or greater) to facilitate growth of CNTs on the metal catalyst particles. The metal catalysts not only play a major role of supporting the growth of CNTs, but can also be used to control the diameter of as-grown CNTs. See, e.g., Cheung, C. L. et al., J. Phys. Chem. B 106, 2429 (2002); and Sato, S. et al., Chem. Phys. Lett. 382, 361 (2003).
The growth of CNTs can take place either on a fixed substrate or in a gas phase using free floating catalysts. For substrate-based growth of CNTs, metal oxide substrates have been used to disperse metal catalysts, where the catalytic properties of metal particles strongly depend upon the interactions between the substrate and the metal particles. In order to minimize the interactions, methods have been developed in which various oxide materials, such as silica and alumina, are inserted between the substrate and the metal particles to achieve a higher yield of CNTs.
Various gas-phase CNT production methods (also referred to as floating catalyst methods), such as laser ablation, wet/dry chemistry and HiPco methods, have been developed in efforts to achieve continuous production of CNTs. However, these processes can involve expensive fabrication approaches (e.g., laser ablation), complex physical conditions (e.g., requiring high pressure conditions of as much as 10 atm or greater) and/or complex chemical processes (e.g., wet/dry chemistry) to produce metal catalysts. In addition, a by-product of trying to increase yield in these processes often results in soot formation. In particular, hydrocarbons are directly decomposed into amorphous carbon at a CNT growth temperature of about 800° C., which can result in a coating of the nanotubes with amorphous carbon (which then necessitates removal of the coating in a subsequent step).
Further, the formation of free-floating CNTs can lead to agglomeration, particularly at high production rates, and the possibility of release of highly mobile aerosols to the environment with potentially adverse environmental and health effects.
Accordingly, it is desirable to provide simple, safe and cost effective methods for producing highly pure CNTs of substantially uniform diameter on the surfaces of metal catalyst particles.